There are various methods for polymerizing a monomer having a vinyl group. Among the methods, an emulsion polymerization method and a suspension polymerization method are important. Since an aqueous medium is used in the methods, the methods are advantageous in that the heat produced by a polymerization reaction is readily removed and it is easy to control the reaction temperature. In addition, the methods are also advantageous in that a product polymer is obtained in the form of a small particle; and therefore, can be readily separated from the reaction mixture, washed and dried.
For more details, in an emulsion polymerization method, a monomer is polymerized using a surfactant and a polymerization initiator in an aqueous medium. A monomer having a vinyl group generally exhibits insolubility or poor solubility in water; however, the monomer is dispersed in an aqueous medium by being entrapped in a micelle composed of surfactants. In such a micelle, the monomer is polymerized by the action of a radical generated from the polymerization initiator. A polymer obtained by an emulsion polymerization method has excellent properties. Specifically, the size of the micelle is about several nanometers, and the polymer is extremely small, that is, as small as from several tens nanometers to several hundreds nanometers. However, the polymer has a high polymerization degree. A dispersion of a polymer obtained by an emulsion polymerization method may be directly used as paint, adhesive or the like in some cases.
In suspension polymerization, a monomer and a polymerization initiator are vigorously stirred to polymerize the monomer in an aqueous medium using a machine to form liquid drops of the monomer generally without using any surfactant. Such a liquid drop is larger than a micelle in an emulsion polymerization method and is usually in a size as large as about 0.01 to 1 mm. There is a fear that the liquid drops may be combined with each other to form a large particle, thereby impairing benefits inherent in suspension polymerization. In order to stabilize the liquid drops, a water-soluble polymer, such as gelatin, starch, polyvinyl alcohol and carboxymethylcellulose, and an insoluble powder, such as calcium carbonate and magnesium carbonate, are added.
However, a polymer obtained by an emulsion polymerization method or a suspension polymerization method has the problem that the above-mentioned surfactant and a liquid drop stabilizer are mixed into the polymer to impair properties inherent in the polymer. Such surfactant and liquid drop stabilizer are also mixed into waste water after the polymer is separated, and it is necessary to reduce amounts of the surfactant and liquid drop stabilizer in the waste water since some of the compounds have a bad influence on the environment. On the one hand, when a monomer that exhibits insolubility or poor solubility in water is subjected to emulsion polymerization and the like, a prescribed amount of a surfactant and the like is required in order to disperse a micelle or a liquid drop of the monomer.
As a biosurfactant, which is a surfactant derived from an organism, surfactin is known. Surfactin has a cyclic peptide structure, and exhibits an excellent surfactant action since the structure of the cyclic moiety that exhibits hydrophilicity is significantly larger than that of a conventional surfactant. For example, a sodium salt thereof therefore has been used for cosmetics and the like as a surfactant (Patent Document 1).
In Patent Documents 2 to 9, surfactin is exemplified as a surfactant used for an emulsion polymerization method.